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%I #12 Mar 04 2018 17:44:45
%S 1,3,2,5,1,7,4,9,1,11,1,13,1,1,8,17,1,19,1,1,1,23,1,25,1,27,1,29,1,31,
%T 16,1,1,1,1,37,1,1,1,41,1,43,1,1,1,47,1,49,1,1,1,53,1,1,1,1,1,59,1,61,
%U 1,1,32,1,1,67,1,1,1,71,1,73,1,1,1,1,1,79,1,81,1,83,1,1,1,1,1,89,1,1,1,1
%N a(n) = smallest positive integer such that lcm(a(1), a(2), ..., a(n)) is a multiple of the n-th triangular number n(n+1)/2.
%e The 7th triangular number is 28; lcm(a(1),a(2),a(3),a(4),a(5),a(6),a(7)) = lcm(1,3,2,5,1,7,4) = 420, which is a multiple of 28. 4 is the smallest value a(7) can take and still have the LCM of the first 7 terms of the sequence be a multiple of 28.
%t a = {1}; l = 1; For[n = 2, n < 80, n++, i = 1; l1 = l; While[ ! Mod[l1, n*(n + 1)/2] == 0, i++; l1 = LCM[l, i]]; AppendTo[a, i]; l = LCM[l, i]]; a (* _Stefan Steinerberger_, Nov 21 2007 *)
%K nonn
%O 1,2
%A _Leroy Quet_, May 29 2007
%E More terms from _Stefan Steinerberger_, Nov 21 2007
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